192 CHEMICAL ENGINEERING the rock from the size at which it comes from the mine or quarry, to a more or less uniform size suitable for secondary reduction. This secondary reduction, whether it be accomplished in one or more stages, may be called crushing. Reduction to finely granular or pulverulent condition may be placed within the class of grinding. These classes are purely arbitrary, but are used as a convenience to distinguish between the departments of rock reduction for which ordinarily machines of different type and character are used. General Principles.—There are several mechanical means by which rock is commercially reduced from one size to another. These include compression, roll action, beam action, impact and attrition. Practically all machines use one of these principles or a combination of two or more of them. Compression has been used for a great many years. It involves the compression or squeezing of rock between two rigid surfaces. One or both of the surfaces may be movable, the rock being fractured by direct compression between two planes. Machines embodying this principle are ordinarily not the most efficient, since they are necessarily reciprocating and require direct application of power without any material leverage. Roll action is the process of reducing rock by means of a hard-faced solid wheel traveling over the rock and breaking it by crushing between the wheel and a rigid surface. While it is a form of compression, it is not the same as the direct-compression method and its results are different. Roll action may be accomplished by means of a wheel traveling over a stationary surface or by means of passing the rock between two wheels wherein the reduction is accomplished. This is a feature of many well-known machines. Beam action accomplishes rock reduction by applying power at an unsupported part of the rock between two rigidly fixed points. This method is a feature of certain crushing machines, which will be mentioned later, and is perhaps one of the most efficient and economical methods of doing this work. Impact is the crushing of rock by means of dropping a weight upon it. Impact crushing may embody features of compression and beam action, but does not involve roll action. It is one of the most efficient means of breaking rock and has been used for years in certain forms and is still used in a modified way. Attrition accomplishes rock reduction by abrasion or by rubbing the rock between two surfaces, one of which may be fixed and the other moving, or both of which may be moving in the same direction but at a different rate of speed, or in different directions. Attrition is one of the most expensive and inconvenient methods of rock reduction, since it reduces the grinding medium to an excessive extent. Attrition is admissible where the rock may be made to grind upon itself, but in practice this has been accomplished only in an extremely limited way. Some machines accomplish the reduction by the employment of only one of these . methods, but the majority of them combine two or more of them. Angle of Nip.—In most crushing machines, the two faces between which the crushing is performed are not parallel, but are inclined to each other. Obviously, this angle, known as the angle of nip, must be sharp enough to prevent any slipping of the rock engaged between the crushing surfaces. If the angle is too obtuse, the rock will slip, or may even be pushed out from between the crushing faces. There is, consequently, a continual effort at compromise between the endeavor to produce an efficient machine, in which the nip angle must be sharp enough to prevent slipping, and the commercial effort to produce a machine having as large an opening as possible for a fixed weight—necessarily involving a more obtuse angle of nip.